Determining the age of surfaces on Mars

Mike Caplinger, Malin Space Science Systems
February 1994

A fundamental problem in planetary science is determining
how the surface of a planet has changed over time. This tells us
something about the dynamics of planetary interiors (for example, how
often volcanoes erupt or how often earthquakes occur) and also
something about the processes that affect planets from without (for
example, how likely it is that a giant asteroid might hit the Earth
and change it radically.)

The simplest way of describing how a surface has changed over time is
to describe the age of each part of that surface. For example, one
could make a map of the surface, color-coding it such that each color
represents a different range of ages. But how are we to determine the
age of a surface? On the Earth, we have easy access to the surface,
and often, to many other older surfaces buried beneath it.
(Obviously, if one surface has buried another, the deeper surface must
be older.) In the Grand Canyon, for example, one can see the
various layers of rock that have been excavated on the canyon walls,
and see the evolution of the surface in the area directly. The
process of dating surfaces by looking at the relationships between
them is called stratigraphy.

In addition, with the development of radiometric dating techniques, we
can determine when a rock was formed or changed state directly, by
measuring the amount of materials produced by radioactive decay within
the rock. This powerful technique has allowed us to determine the
absolute ages of all of the surfaces on the Earth, and those surfaces
on the Moon from which samples have been returned by the Apollo and
Soviet Luna missions.

For the other planets, like Mars, we will be unable to apply
radiometric dating until we can study rocks from their surfaces in a
laboratory. The only tools we can use to explore Mars today are
photographs taken from orbit. Since the first feature of those
photographs one sees are the very large numbers of craters, one asks
"what use can we make of craters to determine something about the
Martian surface?"

In general, the more craters appear on a surface, the older that
surface is. But like most principles in the real world, that rule
must be applied with caution.

Our best theory about how the planets formed is that they were
accreted from smaller bodies, which kept impacting and adding onto the
mass of each planet. Eventually, most of these smaller bodies had hit
the planets, and so the rate of cratering tailed off to almost
zero. The largest bodies (the ones that would form the largest
craters) were used up before the smaller ones, since there were fewer
of the larger ones to start with. So as a rule of thumb, the larger a
crater is, the older it probably is.

We can roughly divide the history of crater formation into three
periods, from oldest to newest:

large and small craters formed

small craters only formed

very few craters formed

Since the planets were formed by the impacts of objects, we would
expect all of the planets to be uniformly covered with both large and
small craters. However, if we take a map of Mars and plot every crater of diameter 100 km or
greater, we get this:

So craters are not uniformly distributed on Mars; instead,
there are a few areas with significant numbers of very large craters
(greater than 300 km in diameter), most of the rest of the southern
highlands have only smaller craters, and all of the northern lowlands
have very few craters.

We can color-code these regions, using red for the most
heavily-cratered areas (the areas with the largest craters), green for
the intermediate areas, and blue for the least-cratered areas.

Very roughly speaking, this is a map of the ages of the surfaces on
Mars. The red surfaces were formed in period 1, the green surfaces
were formed in period 2, and the blue surfaces were formed in period
3. These three periods correspond roughly to the three martian
periods Noachian, Hesperian, and Amazonian (named after regions that
approximate those ages.)

If nothing occurred to change the surface of Mars through time, it
should all look like a Noachian surface. What happened? Since no one
can think of a process that would erase only large craters, there must
be something that erases all craters and "resets" the surface to
smoothness.

This process was probably volcanic in nature: through time, lava flows
buried all of the craters in some areas. Other processes like
subsidence or erosion are also possibilities, but these would have had
to work differently in some areas than others, and this is not likely
-- on the Earth, these forces have destroyed craters everywhere
simultaneously. Unlike volcanic processes, erosional processes
usually happen everywhere at once.

The area with the very largest craters must be the oldest, since it
survives from the Noachian. The area with few very large craters must
have had very large craters on it, but some process must have erased
them. Since that surface still has many smaller craters on it, this
resurfacing must have occurred before the Hesperian ended.

The area in the north with no craters on it must have be resurfaced
after the Hesperian ended, since otherwise many craters would have been
formed on it.

So, in the Noachian Period, Mars was uniformly covered with both large
and small craters. During the Hesperian Period, the Noachian surface
accumulated more small craters and the large craters remained, but the
Hesperian surface was resurfaced and then subsequently accumulated
only small craters. We know that something covered the Amazonian
surface after the end of the Hesperian Period, since if that surface
had been covered before the Hesperian ended, it would have many small
craters on it. But there is no way of knowing when this happened more
precisely; we are unable to tell what the Amazonian surface looked
like before the end of the Hesperian Period.

The impactors that formed the large basins Isidis, Hellas, and Argyre
were so large that there were *never* enough of them to uniformly
cover the planet, even in the Noachian. Examine this detailed map of
Isidis.

Note that it is very smooth, and is about half surrounded by cratered
terrain, and half surrounded by a smooth plain. This suggests that
whatever filled in the Amazonian surface flowed into Isidis, but could
not cover the southern walls of the basin. Remember from the discussion of the crustal dichotomy that the
northern lowlands are lower, and hence easier to cover.

Of course, this discussion is very simplified. There are
stratigraphic relationships (like the boundary of Isidis) that are
used to determine relative ages, in addition to crater counts. The
maps shown here display craters larger than about 100 km, and this can
be done more accurately using many thousands of smaller craters
(recall from our discussion of crater forms
that a 100 km crater is a large crater on Mars.) Also, there are
complex curves for the number of impacts of a particular size: the
division into three periods is fairly arbitrary.

Now note the locations of the major martian volcanoes on the map.

The major volcanoes can be found on the youngest, Amazonian surfaces.
This is to be expected, since the Amazonian surfaces were recently
covered by lava and are thus volcanically active. However, it is a
misconception to believe that the lava that covered these surfaces
came from the visible volcanoes. Instead, these areas are volcanic
plains, usually formed from fissures which are then subsequently
covered by the same lava that flooded the surrounding areas.